JP3697469B2

JP3697469B2 - Perpendicular magnetic recording medium and magnetic storage device using the same

Info

Publication number: JP3697469B2
Application number: JP19200099A
Authority: JP
Inventors: 義幸平山; 正昭二本; 幸雄本多; 敦菊川; 武夫山下; 弘池亀
Original assignee: 株式会社日立グローバルストレージテクノロジーズ
Priority date: 1999-07-06
Filing date: 1999-07-06
Publication date: 2005-09-21
Anticipated expiration: 2019-07-06
Also published as: JP2001023152A

Description

【０００１】
【発明の属する技術分野】
本発明は、コンピュータの補助記憶装置などに用いる磁気記憶装置及びそれに用いる磁気記録媒体に関する。
【０００２】
【従来の技術】
情報化時代の進行により、日常的に扱う情報量は増加の一途を辿っている。これに伴い、磁気記録装置に対する高記録密度化と大容量化の要求が強くなっている。大容量の情報を扱う上では転送速度の高速化も必然的に要求される。磁気記録装置を高記録密度化していった場合、記録ビット当たりの媒体面積が小さくなるため、より高感度の再生ヘッドが使われ、記録媒体には低ノイズ化が求められる。媒体ノイズを低減するためには、磁性膜を薄くして結晶粒の微細化を図ることが効果的である。特に現在の磁気ディスクに用いられている面内磁気記録方式では、隣接ビット間の反磁界を低減してビット境界に生じるノイズを低減するためにも磁性膜の薄膜化が進められている。
【０００３】
【発明が解決しようとする課題】
ただし磁性膜を薄くした場合、記録された磁化が熱的に不安定になるため、保磁力を大きくして記録磁化の安定性を確保しなければならない。特に、高い線記録密度で記録する場合には、ビット間に生じる反磁界に逆らって記録し、しかもその状態を安定に保つ必要があるため、媒体保磁力が大きいことが必須である。媒体保磁力が大きくなると、ヘッドに対して高い記録能力が求められることになり、これが不十分であるとオーバーライトに問題が生じる。このように、高密度化を進める上では、ヘッドの記録能力に限界があることを考えると、媒体の低ノイズ化と熱的安定性確保の両立はたいへん難しい課題である。
【０００４】
一方、磁気記録装置の転送速度を高速化していった場合、媒体への高周波での記録が必要となる。このとき、媒体に印加されるヘッド磁界のパルス幅が短くなることにより、媒体の磁界を反転させるためにより大きな磁界が必要となる。この現象は、例えばIEEE Transactions on Magnetics第３４巻（１９９８年）の１８２８〜１８３２頁に記載されている。反転磁界の増加により、ヘッドに対して高い記録能力が求められることになる。ヘッドの記録能力が限られている場合には、より低い磁界で記録できるように媒体の保磁力を小さくしなければならない。この要求に従って媒体保磁力を小さくした場合には、前述のように高密度記録が難しい。結局、高速転送と高記録密度を両立させた磁気記録装置の実現は現状の技術の改良だけではたいへん難しく、新しい技術の導入が必要である。
【０００５】
高密度化を進めるための新しい技術の有力な候補として、垂直磁気記録方式が検討されている。垂直磁気記録方式には記録密度が高くなるにつれて反磁界が減少するという特徴があり、高密度に記録した場合に記録磁化状態が安定で媒体ノイズも小さく、高密度記録に適した方式であると考えられる。ただし、膜面垂直方向の反磁界のために、低密度に記録されたビットは不安定である。低密度記録状態の熱的安定性を確保するためには、記録膜の磁気異方性を高めるなどして媒体保磁力を高めることが有効である。このことは、Journal of Applied Physics第７９巻（１９９６年）の７９２０〜７９２５頁に記載されている。媒体保磁力を高めた結果、熱的安定性は確保できても、高速転送のための高周波記録は前述の面内記録方式の場合と同様に困難になると予想される。
【０００６】
本発明の目的は、線記録密度３００ｋＦＣＩ以上の高密度記録が可能でかつ毎秒５０メガバイト以上の高速転送が可能な磁気記憶装置及びこれに用いることのできる垂直磁気記録媒体を提供することである。
【０００７】
【課題を解決するための手段】
上記目的の磁気記録媒体は、垂直磁気記録層の残留磁化の飽和磁化に対する比を記録領域内でディスク外周ほど大きく、ディスク最内周で０．７以上とすることにより得られる。また、磁気記録層の保磁力を記録領域内でディスク外周ほど小さく、ディスク最外周で３０００エルステッド以下とすることで本発明の効果が容易に得られる。さらに、垂直磁気記録層の膜厚を記録領域内でディスク外周ほど大きく、記録領域内のディスク最外周で３０ｎｍ以下とすることが好ましく、下地層の膜厚は記録領域内でディスク外周ほど小さくすることが好ましい。
【０００８】
上記目的の磁気記憶装置は、磁気記録媒体と、磁気記録媒体駆動部と、磁気ヘッドと、磁気ヘッド駆動部と、記録再生信号処理系を有する磁気記憶装置において、磁気記録媒体として前述の垂直磁気記録媒体を用い、記録周波数を２５０ＭＨｚ以上、最高線記録密度を３００ｋＦＣＩ以上に設定することにより得られる。
【０００９】
【発明の実施の形態】
本発明による磁気記録媒体の基本的な構成の一例を、図２の断面模式図に示した。図２において２１はガラス、Ｎｉ−Ｐ合金メッキアルミ合金、シリコン、カーボン、チタン合金などの材料で作られたディスク基板である。２２は稠密六方構造のチタンあるいはチタン系合金又は非晶質構造のコバルト系合金などから成る第１の下地層、２３は稠密六方構造のＣｏ−Ｃｒ系合金から成る第２の下地層である。２４はコバルトとクロムを主成分とし、例えばＣｏ−Ｃｒ−Ｔａ、Ｃｏ−Ｃｒ−Ｐｔ、Ｃｏ−Ｃｒ−Ｐｔ−Ｔａ、Ｃｏ−Ｃｒ−Ｎｂ、Ｃｏ−Ｃｒ−Ｐｔ−Ｎｂ、Ｃｏ−Ｃｒ−Ｗなどのような強磁性薄膜を用いた垂直磁気記録層である。２５はカーボン、あるいは窒素、水素、シリコン、ボロンなどを含有したカーボンから成る保護膜と有機系潤滑膜で構成される保護潤滑層である。
【００１０】
作製した磁気記録媒体は、スピンスタンドにおいて記録再生特性の評価を行った。評価の条件としては、ギャップ長０．２μｍ、トラック幅０．９５μｍ、巻線数２０ターンの電磁誘導型ヘッドにより記録し、シールド間隔０．２μｍ、トラック幅０．８５μｍの巨大磁気抵抗効果型ヘッドにより再生を行った。媒体の回転速度や測定半径を広範囲に渡って設定して測定するために、ヘッドのスライダーは磁気スペーシングが回転速度や測定半径に依らず１５ｎｍ前後の一定値になるようなものを選んで用いた。また、高周波記録においてヘッド磁界の立上りの影響を抑えるために、ヘッドアンプとヘッド素子の配線距離を短くするなどの対策を施した。
【００１１】
オーバーライト特性は、次のようにして評価した。線記録密度３５ｋＦＣＩの信号を記録した場合の出力強度Ｎ′をスペクトラムアナライザで測定し、その後ある線記録密度Ｄ（又は記録周波数Ｆ）で重ね書きしたときの３５ｋＦＣＩ信号の消え残り強度Ｎを測定し、線記録密度Ｄ（又は記録周波数Ｆ）のオーバーライト特性として次式の値を求めた。
【００１２】
オーバーライト特性（ｄＢ）＝２０Log（Ｎ′／Ｎ）
媒体Ｓ／Ｎは、線記録密度２ｋＦＣＩの信号を記録した場合の出力Ｓ（μＶ＿０−Ｐ）とある線記録密度（ここでは３００ｋＦＣＩ）の信号を記録した場合の０Ｈｚから４００ＭＨｚの周波数帯域において積算した媒体ノイズＮ（μＶ＿ｒｍｓ）を測定し、次式により求めた。
【００１３】
媒体Ｓ／Ｎ（ｄＢ）＝２０Log（Ｓ／Ｎ）
保磁力、残留磁化及び飽和磁化などの磁気記録媒体の磁気特性は、ディスクから切り出した一辺６ｍｍの正方形の試料について振動試料型磁力計で磁化曲線を測定することにより求めた。
【００１４】
〔実施の形態１〕
基板として表面粗さＲ_aが３ｎｍ以下の直径３．５インチの結晶化ガラス製ディスクを用い、下地層、磁気記録層及び保護層の膜形成は直流マグネトロンスパッタ法により、以下の条件で行った。スパッタ装置内の到達真空度は１×１０^-8トール以下、放電用アルゴンガス圧力は３×１０^-3トール、投入電力は直径６インチのターゲットに対して１ｋＷとした。スパッタ時の基板温度は２６０℃に設定した。垂直磁気記録媒体の第１の下地層として、厚さ３０ｎｍのＴｉ−１０ａｔ％Ｃｒ合金を基板上に直接形成した。第２の下地層として、厚さ２０ｎｍのＣｏ−３５ａｔＣｒ合金を形成した。記録層としては、厚さ２５ｎｍのＣｏ−１９ａｔ％Ｃｒ−１０ａｔ％Ｐｔを形成した。保護潤滑層としては、厚さ５ｎｍの窒素含有カーボン膜と厚さ２ｎｍの有機系潤滑膜を形成した。また、比較検討用の面内磁気記録媒体として、厚さ５０ｎｍのＣｒ−２０ａｔ％Ｔｉ合金を下地として形成した上に、厚さ２０ｎｍのＣｏ−１９ａｔ％Ｃｒ−１０ａｔ％Ｐｔを形成し、上記と同様の保護潤滑層を形成したものを作製した。
【００１５】
保磁力、及び残留磁化の飽和磁化に対する比（以下、これを角形比と呼ぶ）の異なる３種類の垂直磁気記録媒体と保磁力の異なる２種類の面内磁気記録媒体について、媒体の回転速度（１００００ｒｐｍ）及び測定半径（４０．４ｍｍ）を一定にした条件（周速４２．３ｍ／ｓ）で、記録密度を変えてオーバーライト特性の記録周波数依存性を測定した結果、図１に示す結果が得られた。図１において、黒丸で測定値をプロットした曲線１１は保磁力３０００エルステッドで角形比０．９８の垂直磁気記録媒体のオーバーライト特性、測定値を黒い三角でプロットした曲線１２は保磁力３０２０エルステッドで角形比０．７２の垂直磁気記録媒体のオーバーライト特性、測定値を黒い四角でプロットした曲線１３は保磁力３４７０エルステッドで角形比０．９８の垂直磁気記録媒体のオーバーライト特性である。また、測定値を白丸でプロットした曲線１４は保磁力２５２０エルステッドの面内磁気記録媒体のオーバーライト特性、測定値を白い四角でプロットした曲線１５は保磁力３０１０エルステッドの面内磁気記録媒体のオーバーライト特性である。
【００１６】
オーバーライト特性については、信頼性の高い記録再生装置に用いる基準を３０ｄＢ以上と考えて結果を判断した。面内磁気記録媒体の場合は１５０ＭＨｚを超える記録周波数からオーバーライト特性の劣化が始まる。保磁力が２５２０エルステッドの媒体では２５０ＭＨｚ以上の記録周波数が可能であるが、保磁力が３０１０エルステッドの媒体では２５０ＭＨｚの記録周波数で十分なオーバーライト特性が得られない。保磁力２５２０エルステッドの面内磁気記録媒体は、線記録密度３００ｋＦＣＩの記録信号の経時変化を測定すると１０時間で約１０％の減衰が認められ、記録磁化の熱的安定性に問題があることがわかった。面内磁気記録媒体を用いた場合は、２５０ＭＨｚ以上の記録周波数を可能にするために保磁力を小さくすると記録磁化の安定性が損なわれ、逆に記録磁化の安定性を確保するために保磁力を大きくすると高周波記録時のオーバーライト特性が低下するため、この２つの特性を両立させることは困難であると考えられる。
【００１７】
これに対して、垂直磁気記録媒体のオーバーライト特性は記録周波数が２００ＭＨｚを超えても劣化が見られず、劣化が始まってもその変化が面内磁気記録媒体に比べて緩やかである。保磁力が３０００エルステッド程度であれば、２５０ＭＨｚにおいても３０ｄＢ以上のオーバーライト特性が得られ、特に角形比の高い媒体についてはオーバーライト特性の高周波における劣化の割合が小さく、使用可能な周波数帯域が広いことがわかった。以上の結果から、面内磁気記録媒体を用いた場合に生じる高周波記録の問題が、同等の保磁力を持つ垂直磁気記録媒体を用いた場合に抑制されることから、垂直磁気記録媒体が高周波記録に適していることが判明した。
【００１８】
次に同じ媒体について、記録密度（３００ｋＦＣＩ）及び測定半径（４０．４ｍｍ）を一定にした条件（記録周波数２５０ＭＨｚ）で、ディスクの回転数を変えてオーバーライト特性の周速依存性を測定した。媒体の回転数を１００００ｒｐｍから１５０００ｒｐｍに増加させて、周速を４２．３ｍ／ｓから６３．５ｍ／ｓに増加させたとき、２種類の面内磁気記録媒体はいずれもオーバーライト特性が約３ｄＢ劣化した。一方、角形比０．７２及び角形比０．９８の垂直磁気記録媒体はそれぞれ、同様の周速変化に対して約２ｄＢ及び約１ｄＢのオーバーライト特性の劣化に止まった。この結果、周速を大きくしたときに、面内磁気記録媒体は垂直磁気記録媒体に比べて高周波記録がより厳しくなることが明らかとなった。すなわち、周速の大きな条件のもとで高周波記録を行う場合には、角形比の大きい垂直磁気記録媒体を用いるのが有効であることがわかった。
【００１９】
〔実施の形態２〕
実施の形態１と同様の方法で垂直磁気記録媒体を作製した。ただし、ここでは装置に適用できる磁気記録媒体を作製するために、ディスクの外周と内周では周速が異なることを考慮して、磁気記録層の角形比、保磁力などをディスクの半径方向に変化させた種々の媒体を作製して比較した。これらの特性の制御は直流マグネトロンスパッタ用カソードの永久磁石の配置を変えて、ターゲット上のエロージョン領域の位置を変えることで容易に実現できる。
【００２０】
通常の磁気記録媒体の作製においては、エロージョン領域はより均一で広範囲に及ぶように設定することで、ターゲットの使用効率を上げ、ディスクの特性を均一にすることが望ましいと考えられているが、ここではあえて磁気記録層の特性に分布ができるような条件でのスパッタを試みた。エロージョン領域を局在化させた場合にはターゲットと基板の間の距離を変えることによっても磁気記録層の特性に関するディスク半径方向の分布を調節することが可能である。その他にもターゲット組成に分布を持たせることやターゲット表面に細かい起伏を設けてスパッタ粒子の飛び出し角度を調節することによっても、上記のような媒体の作製が可能である。
【００２１】
記録再生特性の評価はディスクの最内周から最外周まで広い範囲に渡って行い、記録周波数２５０ＭＨｚで線記録密度３００ｋＦＣＩの信号を記録したときの媒体Ｓ／Ｎとオーバーライト特性を測定した。この記録再生特性の測定終了後にディスクから磁気特性用試料を切り出し、最内周と最外周を含む５箇所の半径位置において角形比と保磁力の測定を行った。
【００２２】
表１に、ディスクの半径方向における角形比の分布を変えた試料に対する測定結果を示した。保磁力はいずれのディスクも約３０００エルステッドであった。Ｎｏ．１は角形比が半径方向に一定で０．９２と大きな値を有する試料、Ｎｏ．２は最内周側の角形比が０．６８でディスク外周に向かうほど角形比が大くなる試料、Ｎｏ．３は最内周側の角形比が０．７５でディスク外周に向かうほど角形比が大きくなる試料、Ｎｏ．４は角形比がディスク外周に向かうほど小さくなる試料、Ｎｏ．５は角形比がディスク外周に向かうほど小さくなるが全体的にＮｏ．４の試料よりは角形比の大きな試料である。
【００２３】
【表１】

【００２４】
表１から、オーバーライト特性を３０ｄＢ以上にするためには角形比を大きな値にする必要があり、特に外周部における角形比の値を大きくすることが重要であることが明らかとなった。ただし、ディスク全面について角形比を大きくした場合には媒体Ｓ／Ｎを高いレベルにすることは難しく、ディスク全面に渡ってオーバーライト特性と媒体Ｓ／Ｎを同時に高いレベルにするためには角形比を外周ほど大きくすることが必要であることがわかった。ただし、角形比を０．７以下にするとオーバーライト特性が悪くなるため、角形比は少なくとも０．７以上にすることが重要である。
【００２５】
表２に、ディスクの半径方向における保磁力の分布を変えた試料に対する測定結果を示した。角形比は、いずれのディスクも表１のＮｏ．３の試料の分布に準じた値であった。Ｎｏ．１はディスク外周に向かうほど保磁力が大きくなる試料、Ｎｏ．２はディスク外周に向かうほど保磁力が小さくなる試料、Ｎｏ．３はディスク外周に向かうほど保磁力が小さくなるが全体的にＮｏ．２の試料よりは小さな保磁力を有する試料、Ｎｏ．４はディスクの半径方向に保磁力が一定である試料、Ｎｏ．５はディスクの半径方向に保磁力が一定であるが全体的にＮｏ．４の試料よりは大きな保磁力を有する試料である。
【００２６】
【表２】

【００２７】
オーバーライト特性を３０ｄＢ以上にするためには保磁力を小さくする必要があり、特に外周部における保磁力の値を小さくすることが重要であることが明らかとなった。特に３０００エルステッド以下にすることが重要であった。ただし、ディスク全面について保磁力を小さくした場合には媒体Ｓ／Ｎは低下し、ディスク全面に渡ってオーバーライト特性と媒体Ｓ／Ｎを同時に高いレベルにするためには保磁力を外周ほど小さくすることが望ましいことがわかった。
【００２８】
上記のように、ディスクの外周ほど磁気記録膜の角形比を大きくし保磁力を小さくすることは、前記のエロージョン領域の調整などの方法を用いて、結果的に磁気記録層の膜厚をディスク外周ほど大きく、下地層の膜厚を外周ほど小さくなるようにすることで容易に達成できる。具体的には、表２のＮｏ．３の媒体は、磁気記録層の膜厚を最内周で２３ｎｍ、最外周で２５ｎｍとし、第２の下地層であるＣｏ−３５ａｔ％Ｃｒ合金層の膜厚を最内周で２４ｎｍ、最外周で１４ｎｍとして作製したものである。しかも、このような膜厚構成を採用することで、最外周における低線記録密度の記録信号の経時変化を小さく抑える効果もあった。ただし、磁気記録層の膜厚を平均的に増加させた媒体を作製して記録再生特性を測定してみると、磁気記録層膜厚を３０ｎｍより大きくした領域のオーバーライト特性が約２７ｄＢまで劣化したため、磁気記録層膜厚は３０ｎｍ以下にする必要がある。
【００２９】
〔実施の形態３〕
本実施の形態では、垂直磁気記録媒体の第１の下地層として厚さ５００ｎｍのＣｏ−１１ａｔ％Ｎｂ−５ａｔ％Ｚｒ軟磁性膜を用いた。第２の下地層は実施の形態１と同様のＣｏ−３５ａｔ％Ｃｒを用いた。実施の形態２と同様の種々の磁気特性を持つディスク試料を作製し、同様にしてオーバーライト特性と媒体Ｓ／Ｎを測定したところ、全く同じ傾向の結果を得た。
【００３０】
すなわち、図１に示したようなオーバーライト特性の記録周波数依存性を測定すると、オーバーライト特性の高周波領域での劣化の割合が面内磁気記録媒体に比べて緩やかであり、より高い記録周波数まで使用可能であることが分かった。また、角形比を内周側から外周側へ０．７７から０．９７と傾斜を付けた場合に、媒体Ｓ／Ｎが３０．６ｄＢから３１．０ｄＢの範囲になり、オーバーライト特性は３２．０ｄＢから３３．５ｄＢの範囲の高い値を示した。角形比を均一にしたときには媒体Ｓ／Ｎが３０ｄＢを下回り、角形比を０．７未満にしたときにはオーバーライト特性に問題を生じた。保磁力に関しては、内周側から外周側へ３０２０エルステッドから２８５０エルステッドと傾斜を付けた場合に媒体Ｓ／Ｎとオーバーライト特性はいずれの領域でも３１ｄＢ以上の優れた値を示し、ディスクの外周ほど保磁力を小さくすることの有効性が確認された。軟磁性の下地層の有無にかかわらず、本発明の効果があることがわかった。
【００３１】
また、垂直磁気記録膜としてＣｏ−Ｃｒ−Ｔａ、Ｃｏ−Ｃｒ−Ｐｔ−Ｔａ、Ｃｏ−Ｃｒ−Ｎｂ、Ｃｏ−Ｃｒ−Ｐｔ−Ｎｂ、Ｃｏ−Ｃｒ−Ｗを用いた垂直磁気記録媒体に対して実施の形態２と同様の実験を行ったところ、同等の磁気特性が得られた場合には同じ結果となった。
【００３２】
〔実施の形態４〕
実施の形態２において作製した垂直磁気記録媒体の中から表２のＮｏ．３に示した垂直磁気記録媒体を選び、それを図３に概略を示す磁気ディスク装置に組み込み、磁気ディスクの記録再生特性を評価した。この磁気ディスク装置は、図３（ａ）に概略平面図を、図３（ｂ）にそのＡＡ断面図を示すように、磁気ディスク駆動部３２により回転駆動される磁気ディスク３１、磁気ヘッド駆動部３４により保持されて磁気ディスク３１に対して記録および再生を行う磁気ヘッド３３、磁気ヘッド３３の記録信号および再生信号を処理する記録再生信号処理系３５を備える周知の構成の装置である。
【００３３】
ヘッドとしては、実施の形態１で使用したものと同様のものを用い、ヘッドと媒体の間の磁気スペーシングは２０ｎｍ以下となるように調整した。その結果、線記録密度を３００ｋＦＣＩ以上、記録周波数を２５０ＭＨｚ以上に設定することで、毎秒５０メガバイト以上の高速転送が可能であることを確認できた。これに対して、面内磁気記録媒体を用いた場合や表２のＮｏ．１やＮｏ．５に示した媒体を用いた場合は、高速転送での記録再生においてエラーが多発し、装置の信頼性に問題があった。
【００３４】
【発明の効果】
本発明によると、線記録密度３００ｋＦＣＩ以上の高密度記録が可能でかつ毎秒５０メガバイト以上の高速転送が可能な磁気記憶装置及びこれに用いることのできる垂直磁気記録媒体が得られる。
【図面の簡単な説明】
【図１】オーバーライト特性の記録周波数依存性を示す図。
【図２】本発明による垂直磁気記録媒体の基本的構造を示す断面模式図。
【図３】磁気記憶装置の概略図。
【符号の説明】
２１…ディスク基板、２２…第１の下地層、２３…第２の下地層、２４…垂直磁気記録層、２５…保護潤滑層、３１…磁気ディスク、３２…磁気ディスク駆動部、３３…磁気ヘッド、３４…磁気ヘッド駆動部、３５…記録再生信号処理系[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic storage device used for an auxiliary storage device of a computer and the like and a magnetic recording medium used therefor.
[0002]
[Prior art]
With the progress of the information era, the amount of information handled on a daily basis is constantly increasing. Accordingly, there is an increasing demand for higher recording density and larger capacity for magnetic recording devices. In order to handle a large amount of information, it is necessary to increase the transfer speed. When the recording density of a magnetic recording device is increased, the area of the medium per recording bit is reduced. Therefore, a reproducing head with higher sensitivity is used, and the recording medium is required to have low noise. In order to reduce the medium noise, it is effective to make the magnetic film thin and make the crystal grains finer. In particular, in the in-plane magnetic recording method used in the current magnetic disk, the magnetic film is made thinner in order to reduce the demagnetizing field between adjacent bits and reduce noise generated at the bit boundary.
[0003]
[Problems to be solved by the invention]
However, when the magnetic film is thinned, the recorded magnetization becomes thermally unstable, so the coercive force must be increased to ensure the stability of the recorded magnetization. In particular, when recording at a high linear recording density, it is necessary to record against a demagnetizing field generated between bits and to keep the state stable, and therefore it is essential that the medium coercive force is large. When the medium coercive force is increased, a high recording capability is required for the head, and if this is insufficient, a problem occurs in overwriting. As described above, considering that there is a limit to the recording performance of the head in order to increase the density, it is very difficult to achieve both low noise and ensure thermal stability of the medium.
[0004]
On the other hand, when the transfer speed of the magnetic recording apparatus is increased, recording at a high frequency on the medium is required. At this time, since the pulse width of the head magnetic field applied to the medium is shortened, a larger magnetic field is required to reverse the magnetic field of the medium. This phenomenon is described, for example, on pages 1828-1832 of IEEE Transactions on Magnetics Vol. 34 (1998). Due to the increase of the reversal magnetic field, a high recording capability is required for the head. When the recording capability of the head is limited, the coercive force of the medium must be reduced so that recording can be performed with a lower magnetic field. When the medium coercive force is reduced according to this requirement, high density recording is difficult as described above. In the end, realization of a magnetic recording apparatus that achieves both high-speed transfer and high recording density is very difficult only by improving the current technology, and it is necessary to introduce a new technology.
[0005]
Perpendicular magnetic recording has been studied as a promising candidate for a new technology for increasing the density. The perpendicular magnetic recording system has a feature that the demagnetizing field decreases as the recording density increases. When recording at a high density, the recording magnetization state is stable and the medium noise is small, and the system is suitable for high density recording. Conceivable. However, the bit recorded at low density is unstable due to the demagnetizing field in the direction perpendicular to the film surface. In order to ensure the thermal stability in the low density recording state, it is effective to increase the coercive force of the medium by increasing the magnetic anisotropy of the recording film. This is described in pages 7920-7925 of Journal of Applied Physics vol. 79 (1996). As a result of increasing the coercive force of the medium, high-frequency recording for high-speed transfer is expected to be difficult as in the case of the above-described in-plane recording method, even though thermal stability can be ensured.
[0006]
An object of the present invention is to provide a magnetic storage device capable of high-density recording with a linear recording density of 300 kFCI or more and capable of high-speed transfer of 50 megabytes or more per second, and a perpendicular magnetic recording medium that can be used therefor.
[0007]
[Means for Solving the Problems]
The above-mentioned magnetic recording medium can be obtained by setting the ratio of the residual magnetization of the perpendicular magnetic recording layer to the saturation magnetization to be larger in the recording area toward the outer periphery of the disk and 0.7 or more in the innermost periphery of the disk. Further, the effect of the present invention can be easily obtained by setting the coercive force of the magnetic recording layer to be as small as the outer circumference of the disk in the recording area and not more than 3000 Oersted at the outermost circumference of the disk. Further, it is preferable that the thickness of the perpendicular magnetic recording layer is larger toward the outer periphery of the disk in the recording area, and is 30 nm or less at the outermost periphery of the disk in the recording area, and the thickness of the underlayer is decreased toward the outer periphery of the disk in the recording area. It is preferable.
[0008]
The above-mentioned magnetic storage device includes a magnetic recording medium, a magnetic recording medium driving unit, a magnetic head, a magnetic head driving unit, and a recording / reproducing signal processing system. It can be obtained by using a recording medium and setting the recording frequency to 250 MHz or higher and the maximum linear recording density to 300 kFCI or higher.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
An example of the basic configuration of the magnetic recording medium according to the present invention is shown in the schematic sectional view of FIG. In FIG. 2, reference numeral 21 denotes a disk substrate made of a material such as glass, Ni—P alloy plated aluminum alloy, silicon, carbon, or titanium alloy. Reference numeral 22 denotes a first underlayer made of titanium having a dense hexagonal structure, a titanium alloy, or a cobalt alloy having an amorphous structure. Reference numeral 23 denotes a second underlayer made of a Co—Cr alloy having a dense hexagonal structure. 24 is mainly composed of cobalt and chromium, for example, Co-Cr-Ta, Co-Cr-Pt, Co-Cr-Pt-Ta, Co-Cr-Nb, Co-Cr-Pt-Nb, Co-Cr-W. A perpendicular magnetic recording layer using a ferromagnetic thin film such as Reference numeral 25 denotes a protective lubricating layer composed of a protective film made of carbon or carbon containing nitrogen, hydrogen, silicon, boron or the like and an organic lubricating film.
[0010]
The produced magnetic recording medium was evaluated for recording and reproducing characteristics in a spin stand. As evaluation conditions, recording was performed by an electromagnetic induction head having a gap length of 0.2 μm, a track width of 0.95 μm, and the number of windings of 20 turns, and a giant magnetoresistive head having a shield interval of 0.2 μm and a track width of 0.85 μm. Replayed by. In order to set and measure the rotation speed and measurement radius of the medium over a wide range, the head slider should be selected so that the magnetic spacing becomes a constant value around 15 nm regardless of the rotation speed and measurement radius. It was. Further, in order to suppress the influence of the rise of the head magnetic field in high frequency recording, measures such as shortening the wiring distance between the head amplifier and the head element were taken.
[0011]
The overwrite characteristic was evaluated as follows. The output intensity N ′ when a signal with a linear recording density of 35 kFCI is recorded is measured with a spectrum analyzer, and then the unerased intensity N of the 35 kFCI signal when overwritten at a certain linear recording density D (or recording frequency F) is measured. The value of the following equation was obtained as the overwrite characteristic of the linear recording density D (or recording frequency F).
[0012]
Overwrite characteristic (dB) = 20 Log (N '/ N)
The medium S / N is integrated in the frequency band from 0 Hz to 400 MHz when a signal having a linear recording density (300 kFCI in this case) is recorded and an output S (μV_0-P) when a signal having a linear recording density of 2 kFCI is recorded. The medium noise N (μV_rms) was measured and determined by the following equation.
[0013]
Medium S / N (dB) = 20 Log (S / N)
Magnetic characteristics of the magnetic recording medium such as coercive force, residual magnetization, and saturation magnetization were determined by measuring a magnetization curve with a vibrating sample magnetometer on a square sample having a side of 6 mm cut out from the disk.
[0014]
[Embodiment 1]
A crystallized glass disk having _a surface roughness _Ra of 3 nm or less and a diameter of 3.5 inches was used as a substrate, and the underlayer, magnetic recording layer, and protective layer were formed by direct current magnetron sputtering under the following conditions. . The ultimate vacuum in the sputtering apparatus was 1 × 10 ⁻⁸ Torr or less, the discharge argon gas pressure was 3 × 10 ⁻³ Torr, and the input power was 1 kW for a target with a diameter of 6 inches. The substrate temperature during sputtering was set at 260 ° C. A Ti-10 at% Cr alloy having a thickness of 30 nm was directly formed on the substrate as the first underlayer of the perpendicular magnetic recording medium. As the second underlayer, a Co-35atCr alloy with a thickness of 20 nm was formed. As the recording layer, Co-19 at% Cr-10 at% Pt with a thickness of 25 nm was formed. As the protective lubricating layer, a nitrogen-containing carbon film having a thickness of 5 nm and an organic lubricating film having a thickness of 2 nm were formed. Further, as an in-plane magnetic recording medium for comparative study, a Cr-20 at% Ti alloy with a thickness of 50 nm was formed as a base, and then Co-19 at% Cr-10 at% Pt with a thickness of 20 nm was formed. What formed the same protective lubricating layer was produced.
[0015]
For three types of perpendicular magnetic recording media having different coercive force and ratio of residual magnetization to saturation magnetization (hereinafter referred to as square ratio) and two types of in-plane magnetic recording media having different coercive forces, the rotational speed of the medium ( 10000 rpm) and the measurement radius (40.4 mm) were constant (circumferential speed 42.3 m / s), and the recording density was changed to measure the recording frequency dependence of the overwrite characteristics. The result shown in FIG. Obtained. In FIG. 1, a curve 11 in which measured values are plotted with black circles is an overwrite characteristic of a perpendicular magnetic recording medium having a coercive force of 3000 oersted and a squareness ratio of 0.98, and a curve 12 in which measured values are plotted with black triangles is a coercive force of 3020 oersteds. The overwrite characteristic of the perpendicular magnetic recording medium having a squareness ratio of 0.72 and the curve 13 in which the measured values are plotted with black squares are the overwrite characteristics of the perpendicular magnetic recording medium having a coercive force of 3470 oersted and a squareness ratio of 0.98. A curve 14 in which the measured values are plotted with white circles is an overwrite characteristic of the in-plane magnetic recording medium having a coercive force of 2520 oersted, and a curve 15 in which the measured values are plotted in white squares is an overshoot of the in-plane magnetic recording medium having a coercive force of 3010 oersted. Light characteristics.
[0016]
Regarding the overwrite characteristics, the result was judged by considering that the standard used for a highly reliable recording / reproducing apparatus is 30 dB or more. In the case of an in-plane magnetic recording medium, the deterioration of the overwrite characteristics starts from a recording frequency exceeding 150 MHz. A recording frequency of 250 MHz or more is possible with a medium having a coercive force of 2520 oersted, but sufficient overwrite characteristics cannot be obtained at a recording frequency of 250 MHz with a medium having a coercive force of 3010 oersted. An in-plane magnetic recording medium having a coercive force of 2520 Oersted has a problem with the thermal stability of the recording magnetization when a change over time of a recording signal with a linear recording density of 300 kFCI is measured and an attenuation of about 10% is recognized in 10 hours. all right. When an in-plane magnetic recording medium is used, if the coercive force is decreased to enable a recording frequency of 250 MHz or higher, the stability of the recording magnetization is impaired, and conversely, the coercive force is ensured to ensure the stability of the recording magnetization. If the value is increased, the overwrite characteristic at the time of high-frequency recording is lowered, so that it is considered difficult to achieve both of these characteristics.
[0017]
On the other hand, the overwrite characteristics of the perpendicular magnetic recording medium are not deteriorated even when the recording frequency exceeds 200 MHz, and even when the deterioration starts, the change is more gradual than the in-plane magnetic recording medium. If the coercive force is about 3000 Oersteds, an overwrite characteristic of 30 dB or more can be obtained even at 250 MHz, and particularly for a medium with a high squareness ratio, the deterioration rate of the overwrite characteristic at a high frequency is small and the usable frequency band is wide. I understand. From the above results, the problem of high-frequency recording that occurs when using an in-plane magnetic recording medium is suppressed when a perpendicular magnetic recording medium having the same coercive force is used. Turned out to be suitable.
[0018]
Next, with respect to the same medium, the peripheral speed dependency of the overwrite characteristic was measured by changing the number of revolutions of the disk under the condition that the recording density (300 kFCI) and the measurement radius (40.4 mm) were constant (recording frequency 250 MHz). When the rotational speed of the medium is increased from 10,000 rpm to 15000 rpm and the peripheral speed is increased from 42.3 m / s to 63.5 m / s, the two types of in-plane magnetic recording media have an overwrite characteristic of about 3 dB. Deteriorated. On the other hand, the perpendicular magnetic recording media having a squareness ratio of 0.72 and a squareness ratio of 0.98 stopped to deteriorate the overwrite characteristics of about 2 dB and about 1 dB, respectively, with respect to the same peripheral speed change. As a result, it has been clarified that when the peripheral speed is increased, the in-plane magnetic recording medium is more severe in high frequency recording than the perpendicular magnetic recording medium. That is, it was found that it is effective to use a perpendicular magnetic recording medium having a large squareness ratio when performing high-frequency recording under a condition with a large peripheral speed.
[0019]
[Embodiment 2]
A perpendicular magnetic recording medium was manufactured by the same method as in the first embodiment. However, here, in order to produce a magnetic recording medium applicable to the apparatus, the square ratio of the magnetic recording layer, the coercive force, etc. are set in the radial direction of the disk in consideration of the fact that the peripheral speed differs between the outer circumference and the inner circumference of the disk. Various changed media were made and compared. Control of these characteristics can be easily realized by changing the position of the erosion region on the target by changing the arrangement of the permanent magnets of the cathode for DC magnetron sputtering.
[0020]
In the production of ordinary magnetic recording media, it is considered desirable to increase the use efficiency of the target and make the characteristics of the disk uniform by setting the erosion area to be more uniform and wide. In this case, sputtering was attempted under the condition that the characteristics of the magnetic recording layer can be distributed. When the erosion region is localized, it is possible to adjust the distribution in the disk radial direction with respect to the characteristics of the magnetic recording layer by changing the distance between the target and the substrate. In addition, the medium as described above can also be produced by providing a distribution in the target composition or adjusting the popping-out angle of the sputtered particles by providing fine undulations on the target surface.
[0021]
The recording / reproduction characteristics were evaluated over a wide range from the innermost circumference to the outermost circumference of the disk, and the medium S / N and overwrite characteristics when a signal with a linear recording density of 300 kFCI was recorded at a recording frequency of 250 MHz were measured. After the measurement of the recording / reproducing characteristics, a sample for magnetic characteristics was cut out from the disk, and the squareness ratio and coercive force were measured at five radial positions including the innermost circumference and the outermost circumference.
[0022]
Table 1 shows the measurement results for the samples in which the distribution of the squareness ratio in the radial direction of the disk is changed. The coercivity was about 3000 Oersted for all disks. No. No. 1 is a sample having a squareness ratio constant in the radial direction and a large value of 0.92, No. 1 No. 2 is a sample in which the squareness ratio on the innermost circumference side is 0.68, and the squareness ratio increases toward the outer circumference of the disk. No. 3 is a sample in which the squareness ratio on the innermost circumference side is 0.75, and the squareness ratio increases toward the outer circumference of the disk. No. 4 is a sample whose squareness ratio decreases toward the outer periphery of the disk, No. 4 No. 5 decreases as the squareness ratio increases toward the outer periphery of the disk. It is a sample having a larger squareness ratio than the sample of 4.
[0023]
[Table 1]

[0024]
From Table 1, it has been clarified that it is necessary to increase the squareness ratio in order to make the overwrite characteristic 30 dB or more, and it is particularly important to increase the value of the squareness ratio in the outer peripheral portion. However, when the squareness ratio is increased on the entire disk surface, it is difficult to increase the medium S / N level. To increase the overwrite characteristic and the medium S / N level simultaneously on the entire disk surface, the rectangular ratio is required. It has been found that it is necessary to increase the outer diameter of the outer periphery. However, when the squareness ratio is 0.7 or less, the overwrite characteristic is deteriorated. Therefore, it is important that the squareness ratio is at least 0.7 or more.
[0025]
Table 2 shows the measurement results for samples with different coercive force distributions in the radial direction of the disk. The squareness ratio of each disk is No. 1 in Table 1. It was a value according to the distribution of 3 samples. No. No. 1 is a sample whose coercive force increases toward the outer periphery of the disk. No. 2 is a sample whose coercive force decreases toward the outer periphery of the disk. No. 3 has a smaller coercive force toward the outer periphery of the disk, but overall, No. 3 No. 2 sample having a smaller coercive force than the sample No. 2 No. 4 is a sample having a constant coercive force in the radial direction of the disk. No. 5 has a constant coercive force in the radial direction of the disk, but overall No. It is a sample having a coercive force larger than that of sample 4.
[0026]
[Table 2]

[0027]
In order to make the overwrite characteristic 30 dB or more, it is necessary to reduce the coercive force, and it has become clear that it is particularly important to reduce the value of the coercive force in the outer peripheral portion. In particular, it was important to make it 3000 or less. However, when the coercive force is reduced over the entire disk surface, the medium S / N decreases, and the coercive force is decreased toward the outer periphery in order to simultaneously increase the overwrite characteristics and the medium S / N over the entire disk surface. I found it desirable.
[0028]
As described above, increasing the squareness ratio of the magnetic recording film and reducing the coercive force toward the outer periphery of the disk results in the film thickness of the magnetic recording layer being reduced by using the method of adjusting the erosion area. This can be easily achieved by increasing the outer periphery and decreasing the thickness of the underlayer as the outer periphery. Specifically, No. 2 in Table 2 is used. In the medium 3, the thickness of the magnetic recording layer is 23 nm at the innermost circumference and 25 nm at the outermost circumference, and the thickness of the Co-35 at% Cr alloy layer as the second underlayer is 24 nm at the innermost circumference. And 14 nm. In addition, by adopting such a film thickness configuration, there is also an effect of suppressing a change with time of a recording signal having a low linear recording density at the outermost periphery. However, when the recording / reproducing characteristics are measured by producing a medium having an average increased thickness of the magnetic recording layer, the overwrite characteristics in a region where the thickness of the magnetic recording layer is larger than 30 nm is deteriorated to about 27 dB. Therefore, the thickness of the magnetic recording layer needs to be 30 nm or less.
[0029]
[Embodiment 3]
In the present embodiment, a Co-11 at% Nb-5 at% Zr soft magnetic film having a thickness of 500 nm is used as the first underlayer of the perpendicular magnetic recording medium. As the second underlayer, Co-35 at% Cr similar to that in Embodiment 1 was used. When disk samples having various magnetic characteristics similar to those of the second embodiment were produced and the overwrite characteristics and the medium S / N were measured in the same manner, results having exactly the same tendency were obtained.
[0030]
That is, when the dependence of the overwrite characteristic on the recording frequency as shown in FIG. 1 is measured, the rate of deterioration of the overwrite characteristic in the high frequency region is gradual compared to the in-plane magnetic recording medium, and even higher recording frequency. It turned out to be usable. Further, when the squareness ratio is inclined from 0.77 to 0.97 from the inner peripheral side to the outer peripheral side, the medium S / N is in the range of 30.6 dB to 31.0 dB, and the overwrite characteristic is 32. A high value in the range of 0 dB to 33.5 dB was shown. When the squareness ratio was made uniform, the medium S / N was less than 30 dB, and when the squareness ratio was less than 0.7, a problem occurred in the overwrite characteristics. With respect to the coercive force, the medium S / N and the overwrite characteristics show excellent values of 31 dB or more in any region when the gradient is applied from 3020 Oersted to 2850 Oersted from the inner circumference side to the outer circumference side. The effectiveness of reducing the coercive force was confirmed. It was found that the present invention is effective regardless of the presence or absence of a soft magnetic underlayer.
[0031]
For perpendicular magnetic recording media using Co—Cr—Ta, Co—Cr—Pt—Ta, Co—Cr—Nb, Co—Cr—Pt—Nb, and Co—Cr—W as perpendicular magnetic recording films. When an experiment similar to that of the second embodiment was performed, the same result was obtained when an equivalent magnetic characteristic was obtained.
[0032]
[Embodiment 4]
Among the perpendicular magnetic recording media manufactured in the second embodiment, No. 1 in Table 2 was obtained. The perpendicular magnetic recording medium shown in FIG. 3 was selected and incorporated in the magnetic disk apparatus schematically shown in FIG. 3, and the recording / reproducing characteristics of the magnetic disk were evaluated. As shown in FIG. 3A and FIG. 3B, a cross-sectional view taken along line AA in FIG. 3A, this magnetic disk apparatus is provided with a magnetic disk 31 rotated by a magnetic disk drive unit 32, a magnetic head drive unit. 34 is a device having a known configuration including a magnetic head 33 which is held by a magnetic head 33 and performs recording and reproduction with respect to the magnetic disk 31, and a recording / reproduction signal processing system 35 which processes a recording signal and a reproduction signal of the magnetic head 33.
[0033]
A head similar to that used in Embodiment 1 was used, and the magnetic spacing between the head and the medium was adjusted to 20 nm or less. As a result, it was confirmed that by setting the linear recording density to 300 kFCI or more and the recording frequency to 250 MHz or more, high-speed transfer of 50 megabytes or more per second was possible. On the other hand, when the in-plane magnetic recording medium is used and 1 and No. When the medium shown in FIG. 5 was used, errors frequently occurred in recording and reproduction at high speed transfer, and there was a problem in reliability of the apparatus.
[0034]
【The invention's effect】
According to the present invention, a magnetic storage device capable of high-density recording with a linear recording density of 300 kFCI or higher and capable of high-speed transfer of 50 megabytes or more per second, and a perpendicular magnetic recording medium that can be used therefor are obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing recording frequency dependence of overwrite characteristics.
FIG. 2 is a schematic cross-sectional view showing the basic structure of a perpendicular magnetic recording medium according to the present invention.
FIG. 3 is a schematic diagram of a magnetic storage device.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 21 ... Disk substrate, 22 ... 1st base layer, 23 ... 2nd base layer, 24 ... Perpendicular magnetic recording layer, 25 ... Protective lubricating layer, 31 ... Magnetic disk, 32 ... Magnetic disk drive part, 33 ... Magnetic head 34 ... Magnetic head drive unit, 35 ... Recording / reproduction signal processing system

Claims

And a base layer and a perpendicular magnetic recording layer is formed by laminating on a disk-shaped substrate, wherein the ratio of the saturation magnetization of the residual magnetization of the perpendicular magnetic recording layer is rather large as the outer periphery of the disk in the recording area, the perpendicular magnetic recording layer The ratio of the residual magnetization to the saturation magnetization is 0.7 or more at the innermost circumference of the disk in the recording area, and the coercive force of the perpendicular magnetic recording layer is smaller toward the outer circumference of the disk in the recording area. The perpendicular magnetic recording medium is characterized by being 3000 oersted or less .

The vertical thickness of the magnetic recording layer is larger as the disk outer periphery in the recording area, perpendicular magnetic recording medium according to claim 1, wherein the disk outermost circumference of the recording area is 30nm or less.

3. The perpendicular magnetic recording medium according to claim 2, wherein the thickness of the underlayer is smaller toward the outer periphery of the disk in the recording area.

A magnetic recording medium, a magnetic recording medium drive unit, a magnetic head, a magnetic head drive unit, the magnetic storage device having a recording and reproducing signal processing system, according to any one of claims 1 to 3 as the magnetic recording medium And a perpendicular magnetic recording medium having a recording frequency of 250 MHz or more and a maximum linear recording density of 300 kFCI or more.